Drilling technology has an increasing role in the future economy of crude oil and natural gas. Especially deep drilling has a large potential when the natural gas reserves are considered. A large portion of the remaining natural gas reserves exist at deeper depths where oil is no longer the primary target. Considerable deeper drilling of present oil wells may be required to explore deeper gas fields.
In the recent past, several investigators have dedicated their energy and time to adapt new and unusual techniques to solve the various problems encountered in oil well drilling. The alternative techniques included are explosives, percussion, chemical, water jet, abrasive fluid jet, melting, and thermal spallation methods. Evaluations were based on specific energy per foot drilled, penetration rates, compatibility with cuttings removal and well control operations. Each of these alternative methods have at least one major deficiency, namely explosives create a uncontrollable hole size and cutting removal is more difficult. Ballistic percussion is very energy efficient in large scale mining but difficult to implement and less efficient in performance in the small bore, mud filled deep wells. Thermal spallation does not work well in most sedimentary rocks. Rock comminution theory, along with current drilling practice, indicates better performance for drag and shear cutting methods over roller cone percussion bits for breaking rock as rock stress increases with depth.
Use of slim hole drilling and down hole motors has been increasing for several reasons. Slim holes drill quickly and cost less due to lower costs of bits, casing, rigs, mud, and crews. Down hole motors offer more efficient use of energy and better directional control. Factors important to the use and development of these techniques are drilling string strength, directional control for top driven systems, down hole motor service life, and removal of abrasive from mud before it is re-circulated.
David A. Summers and Richard L. Henry of University of Missouri-Rolla have done studies in the laboratory to evaluate the specific energy requirements for water jet cutting of rock with and without mechanical assistance. These studies are described in Water Jet Cutting of Rock With and Without Mechanical Assistance, Paper No. 3533, SPE 46th Annual Fall Meeting, New Orleans, Oct. 3-6, 1971. The relative efficiencies were evaluated in the pressure range of 5,000 psi to 25,000 psi. Water jets were alone utilized in the first case for rock removal, and in the second water jets were allowed to cut slots and the ridges were removed with a mechanical cutter. A high pressure jet nozzle of diameter 0.023 in. was used on samples of Berea sandstone and Indiana limestone.
A.W. lyoho discusses the various applications of high pressure water jet rock removal technology in geothermal wells, underground mining, drilling in coal, environmental applications, horizontal wells, re-entry wells, coiled tubing applications, and enhanced oil recovery in Petroleum Applications of . . . . The use of high pressure jet technology requires sophisticated equipment, control systems, and expensive delivery system. So the recent trend in research is to combine relatively low pressure jets (5000 psi) with mechanical drilling. The water jets used are abrasive laden to improve the performance and rate of penetration. The energy of the high pressure jet is rapidly reduced as the depth increases due to the need to overcome the energy of the fluid that exists the hole. This problem was solved by introducing the jets in pulses and directing the rotating jet in a slightly offset angle.
The high pressure jet generally tries to enter any mechanical flaws in the rock face and further weaken the rock face and remove the dislodged particles. Other mechanisms of rock removal relevant to high pressure jet drilling are discussed. Results prove that with mastery attained in this emerging technology, reliable equipment, and experience, the area of horizontal oil well drilling will be a potential candidate. The tool is well suitable for drilling short radius wells and is also suitable for drilling a number of radials from a single well bore. The main reason for this is the necessity of low weight on bit due to the combined action of two processes: viz. mechanical drilling and the abrasive laden high pressure jet. The constraints on weight on bit for the directional control in deviated and horizontal drilling is completely eliminated. The low weight on bit enables a better control on the hole angle. Bechtel has attempted to drill a lateral hole off a previously drilled vertical hole. Coiled tubing was used with jet heads. The tool was advanced not by the weight on bit but by jet pressure and injection force.
Alan D. Peters of Penetrators, Inc. discusses another interesting application, of drilling small diameter radial holes to penetrate the damaged zone around a producing well bore in his article, The Lance Formation Penetration System, Southwestern Petroleum Short Course, 1990. The tool named Lanse.SM. Formation Penetrator activates itself down hole when pressurized. With the application of 10,000 psi, a steel punch from the tool punches a hole in the casing. Immediately, a jet from the lance provided cuts through the cement sheath and proceeds drilling a small pilot hole into the damaged formation. As drilling progresses, the lance extends horizontally into the formation up to a length of 10 feet. The discharge used was 20 gallons per minute of clean fluid. Once drilling is over the pressure is reduced to retract the lance into the tool. Peters' paper again proves the tremendous potential of jet cutting in the drilling industry.
Mike Cure of Grace Drilling Company and Pete Fontana of FlowDril Corp. have commercially realized a technology to combine jet and mechanical drilling. This technology is described in the Oil & Gas Journal, Mar. 11, 1991, pp. 56-66. The system consisted of a ultra high pressure pump on the surface to pressurize 20-30 gallons of mud a minute and deliver it to special high pressure nozzles on the bit through a small diameter piping running down inside the drill pipe and drill collars from the surface to the bottom. Considerable changes in surface equipment and down hole tubular were necessary for this system. The gooseneck, swivel, kelly, drill pipes and drill collars were modified to accommodate the dual pipe system.
The Cure set up involves an elaborate arrangement of surface equipment. FIG. 1 gives a detailed ideal of the setup. The surface equipment mainly consists of a drilling fluid condition equipment, ultra high pressure pumps, isolator, and the necessary piping to delivery the clean high pressure mud to the drill string. The drill string consists of the same equipment as found on a conventional rig namely, swivel, kelly, drill pipe, drill collars, stabilizers, subs, and other bottom hole assemblies. The swivel, kelly, drill pipe, drill collars, stabilizers, subs and other bottom hole assemblies are modified to accommodate the dual pipe system. In other words all these pieces of equipment accommodate an inner high pressure tubing of diameter 1.625" OD. This tubing runs down the center of the drill string from the surface to the bit, and exits to special high pressure nozzles situated on the bit. The bit accommodates both, the three conventional nozzles and the special high pressure nozzles for jet cutting. Only 20 to 30 gal/min of mud is pressurized with the ultra high pressure pump for the sake of jet drilling. Approximately 400 gal/min of mud is pumped into the drill string with the conventional mud pumps. Therefore two mud streams assist, one the 20 to 30 gal/min of clean pressurized mud, and the other 400 gal per min of regular mud. The high pressure mud flows through the inner high pressure tubing and the regular mud flows through the annulus between the drill string and the inner tubing. Both of these streams on exiting through their respective nozzles mix together in the annulus and return to the surface with the cutting. The high pressure jet would assist the normal mechanical drilling which is due to rotating drill string and weight on bit.
The drilling fluid conditioning equipment conditions the mud and cleans it of all abrasive materials. This clean fluid ensures long life of the ultra high pressure pump. The ultra high pressure pump is a critical part because of the amount of pressure it generates (35,000 psi). This clean fluid prevents the mud cut of the pump which is otherwise a normal occurrence in the conventional mud pumps. This cleaning equipment pressurizes only 20 to 30 gal/min of mud since this is the quantity required for jetting action.
The ultra high pressure pump as shown in FIG. 3.2 is truck mounted and is mechanical crankshaft driven. These pumps require 600-800 bhp. Because of the high pressure, any suspended abrasive particle is removed by the cleaning equipment before it passes through the pump.
The drill string contains a inner tubing to deliver the high pressure mud from the surface to the bit. This tubing is made of berryllium--copper alloy and the centralizer design ensures the tubing's entire stabilization in the 5 inch drill pipe. Berryllium--copper alloy was chosen as it is more resistant to chloride stress cracking. The life of this tubing goes beyond that of the drill pipe. The connections are so designed that, when the drill pipe connections are made, automatically the inner tubing gets connected.
In spite of the impressive results obtained in these test wells some drawbacks do lie in this system. Some of the important disadvantages in this system are listed as follows:
1. Increased Rig installation costs. The surface equipment necessary viz. the drilling fluid condition equipment, the ultra high pressure pump, the modified swivel, tubular with the dual pipe, etc., are the extra investments a drilling company has to make. PA1 2. The modified swivel has proven itself operational for only 160 hours at 20,000 psi to 30,000 psi. PA1 3. Modifying wire line observation tools and all down hole tools like positive displacement motors, turbines, jars, etc., because of the dual pipes involved in this system will be very tedious and expensive. PA1 4. Fishing operations will be more complicated. PA1 5. The concentric high pressure tube within the drill pipe is laborious to install. PA1 6. The stab seal design of the inner conduit cannot be completely dependable.
These drawbacks compel costly changes and modifications in drilling operations like fishing, deviation measurements, MWD, turbo drilling and horizontal drilling.